Magnetoresistive element

ABSTRACT

A magnetoresistive element includes a first magnetic layer which includes a first surface and a second surface and has a first standard electrode potential, a second magnetic layer, a barrier layer which is provided between the second magnetic layer and the first surface of the first magnetic layer, and a nonmagnetic cap layer which contacts the second surface of the first magnetic layer and is formed from an alloy of a first metal material and a second metal material, the first metal material having a second standard electrode potential lower than the first standard electrode potential, the second metal material having a third standard electrode potential higher than the first standard electrode potential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2005-183718, filed Jun. 23, 2005,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetoresistive element having analloy cap layer and a magnetic random access memory (magnetoresistiverandom access memory) (MRAM).

2. Description of the Related Art

A magnetic random access memory (MRAM) is a memory device which usesmagnetic elements having a magnetoresistance effect as cell units tostore information. Magnetic random access memories have received a greatdeal of attention as next-generation memory devices featuring high-speedoperation, large capacity, and nonvolatility. The magnetoresistanceeffect is a phenomenon that when a magnetic field is applied to aferromagnetic material, the electrical resistance changes in accordancewith the magnetization direction in the ferromagnetic material. Amagnetic random access memory can be operated as a memory device (MRAM)by recording information by using such a magnetization direction in aferromagnetic material and reading out the information on the basis ofthe magnitude of electrical resistance corresponding to the information.

In recent years, in a ferromagnetic tunnel junction including a sandwichstructure having an insulating layer (tunnel barrier layer: to bereferred to as a barrier layer hereinafter) inserted between twoferromagnetic layers, a magnetoresistive ratio (MR ratio) of 20% or morecan be obtained by a tunnel magneto-resistance (TMR) effect. With thisas a momentum, magnetic random access memories which use magnetic tunneljunction (MTJ) elements using the tunnel magneto-resistance effect havebeen receiving expectation and attention.

When an MTJ element is used in a magnetic random access memory, one oftwo ferromagnetic layers sandwiching a barrier layer is formed as amagnetization reference layer by using a magnetization pinned layer inwhich the magnetization direction is fixed and does not change. Theother ferromagnetic layer is formed as a free layer by using amagnetization free layer in which the magnetization direction readilyreverses. When a state wherein the magnetization direction in thereference layer and that in the free layer are parallel and a statewherein the magnetization directions are anti-parallel are made tocorrespond to binary numbers of “0” and “1”, respectively, informationcan be stored. When the magnetizations are parallel, the resistance ofthe barrier layer is low, and the tunnel current is large, as comparedto the anti-parallel state. Recording information is written byreversing the magnetization direction in the free layer by an inducedmagnetization which is generated when a current flows to a writeinterconnection provided near the MTJ element. Recorded information isread out by detecting a change in resistance by the TMR effect. Hence,the free layer preferably has a high resistance change ratio (MR ratio)by the TMR effect.

When the free layer is located above the magnetization pinned layer, acap layer is often formed between the free layer and an upperinterconnection layer or an etching mask. The upper interconnectionlayer or etching mask sometimes also serves as the cap layer. The mainrole of the cap layer is to prevent any degradation in magnetization ofthe free layer, which would be caused by element diffusion from theupper layer in the heating process or process damage in the upperinterconnection layer formation process. In addition, increasing thethermal stability by preventing element diffusion from the cap layeritself to the free layer, and preventing any decrease in MR ratio byinteraction with the free layer are necessary. However, no method ofsatisfying both of them has been reported yet.

As described above, in the cap layer of the MTJ element used in theconventional magnetic random access memory, improvement of the thermalstability by preventing element diffusion from the cap layer to themagnetic layer and prevention of any decrease in MR ratio caused byabnormal oxidation of the free layer are not implemented simultaneously.

References of prior arts related to the present invention are Jpn. Pat.Appln. KOKAI Publication No. 2005-032780, U.S. Patent ApplicationPublication No. 2005/0008849, and Jpn. Pat. Appln. KOKAI PublicationNos. 2002-208119, 2002-050011, 2001-331908, and 2004-172599.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda magnetoresistive element comprising a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential, a second magnetic layer, a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer, and a nonmagnetic cap layer which contacts thesecond surface of the first magnetic layer and is formed from an alloyof a first metal material and a second metal material, the first metalmaterial having a second standard electrode potential lower than thefirst standard electrode potential, the second metal material having athird standard electrode potential higher than the first standardelectrode potential.

According to a second aspect of the present invention, there is provideda magnetoresistive element comprising a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential, a second magnetic layer, a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer, a nonmagnetic cap layer which is arranged on aside of the second surface of the first magnetic layer and is formedfrom an alloy of a first metal material and a second metal material, thefirst metal material having a second standard electrode potential lowerthan the first standard electrode potential, the second metal materialhaving a third standard electrode potential higher than the firststandard electrode potential, and a diffusion suppressing layer whichsuppresses diffusion from the nonmagnetic cap layer to the firstmagnetic layer and is provided between the nonmagnetic cap layer and thefirst magnetic layer, the diffusion suppressing layer containing one ofa metal oxide, a metal nitride, and a metal oxynitride.

According to a third aspect of the present invention, there is provideda magnetoresistive element comprising a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential, a second magnetic layer, a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer, and a nonmagnetic cap layer which contacts thesecond surface of the first magnetic layer and is formed from an alloyof a magnetic material and a metal material, the metal material having asecond standard electrode potential lower than the first standardelectrode potential.

According to a fourth aspect of the present invention, there is provideda magnetoresistive element comprising a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential, a second magnetic layer, a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer, a nonmagnetic cap layer which is arranged on aside of the second surface of the first magnetic layer and is formedfrom an alloy of a magnetic material and a metal material, the metalmaterial having a second standard electrode potential lower than thefirst standard electrode potential, and a diffusion suppressing layerwhich suppresses diffusion from the nonmagnetic cap layer to the firstmagnetic layer and is provided between the nonmagnetic cap layer and thefirst magnetic layer, the diffusion suppressing layer containing one ofa metal oxide, a metal nitride, and a metal oxynitride.

According to a fifth aspect of the present invention, there is provideda magnetic random access memory comprising a magnetoresistive element ofany one of the first to fourth aspects as a memory element.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A is a graph showing comparison of the barrier oxidation timedependence of the barrier resistance of an MTJ element between an Ru caplayer and a Ta cap layer;

FIG. 1B is a graph showing comparison of the barrier oxidation timedependence of the barrier resistance change of an MTJ element between anRu cap layer and a Ta cap layer;

FIG. 1C is a graph showing comparison of the barrier oxidation timedependence of the magnetoresistive ratio (MR ratio) of an MTJ elementbetween an Ru cap layer and a Ta cap layer;

FIG. 2 is a table showing the standard electrode potentials and reactionformulas of various metal elements;

FIGS. 3A and 3B are views showing models so as to explain the differencein barrier resistance and magnetoresistive ratio between cap layers;

FIG. 4 is a schematic view showing an MTJ element according to the firstembodiment of the present invention;

FIG. 5 is a graph showing the Ru—Ta or Ru—Cr composition dependence ofthe magnetization change in a free layer upon annealing when an Ru—Ta orRu—Cr alloy cap layer is used as the alloy cap layer according to thefirst embodiment of the present invention;

FIG. 6 is a graph showing the Ru—Ta composition dependence of thebarrier resistance and magnetoresistive ratio of an MTJ element when anRu—Ta alloy cap layer is used as the alloy cap layer according to thefirst embodiment of the present invention;

FIG. 7 is a graph showing the metal composition dependence of thestandard electrode potential (ionization tendency) of an Ru—Ta or Ru—Cralloy cap layer when the Ru—Ta or Ru—Cr alloy cap layer is used as thealloy cap layer according to the first embodiment of the presentinvention;

FIG. 8 is a graph showing the relationship between the metal compositionratio and the standard electrode potential of an alloy cap layer made ofRu—Ti, Ru—Hf, Ru—Zr, or Ru—Nb according to the first embodiment of thepresent invention;

FIG. 9 is a graph showing the relationship between the metal compositionratio and the standard electrode potential of an alloy cap layer made ofCu—Ti, Cu—Hf, Cu—Zr, or Cu—Nb according to the first embodiment of thepresent invention;

FIG. 10 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Rh—Ti, Rh—Hf, Rh—Zr, or Rh—Nb according to the firstembodiment of the present invention;

FIG. 11 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Ag—Ti, Ag—Hf, Ag—Zr, or Ag—Nb according to the firstembodiment of the present invention;

FIG. 12 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Pd—Ti, Pd—Hf, Pd—Zr, or Pd—Nb according to the firstembodiment of the present invention;

FIG. 13 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Ir—Ti, Ir—Hf, Ir—Zr, or Ir—Nb according to the firstembodiment of the present invention;

FIG. 14 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Pt—Ti, Pt—Hf, Pt—Zr, or Pt—Nb according to the firstembodiment of the present invention;

FIG. 15 is a graph showing the relationship between the metalcomposition ratio and the standard electrode potential of an alloy caplayer made of Au—Ti, Au—Hf, Au—Zr, or Au—Nb according to the firstembodiment of the present invention;

FIG. 16 is a sectional view showing an MTJ element according to Example1 of the present invention;

FIG. 17 is a sectional view showing an MTJ element according to Example2 of the present invention;

FIG. 18 is a sectional view showing an MTJ element according to Example3 of the present invention;

FIGS. 19A, 19B, 19C, 19D, 19E, and 19F are sectional views showing stepsin manufacturing a portion near a memory cell of an MTJ elementaccording to Example 4 of the present invention;

FIG. 20 is a schematic view showing an MTJ element according to thesecond embodiment of the present invention;

FIG. 21 is a graph showing the NiFe—Zr composition dependence of thebarrier resistance and magnetoresistive ratio of an MTJ element when anNi—Fe—Zr alloy cap layer is used as an alloy cap layer according to thesecond embodiment of the present invention;

FIG. 22 is a schematic view showing an MTJ element according to thethird embodiment of the present invention;

FIGS. 23A and 23B are views showing select transistor memory cells of amagnetic random access memory according to an embodiment of the presentinvention, in which FIG. 23A is a circuit diagram showing a memory cellarray, and FIG. 23B is a sectional view showing one cell;

FIGS. 24A and 24B are views showing select diode memory cells of amagnetic random access memory according to an embodiment of the presentinvention, in which FIG. 24A is a circuit diagram showing a memory cellarray, and FIG. 24B is a sectional view showing one cell;

FIGS. 25A and 25B are views showing cross-point memory cells of amagnetic random access memory according to an embodiment of the presentinvention, in which FIG. 25A is a circuit diagram showing a memory cellarray, and FIG. 25B is a sectional view showing one cell; and

FIG. 26 is a plan view showing a toggle memory cell of a magnetic randomaccess memory according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors made the following examinations to simultaneouslyimprove thermal stability and MR ratio in a magnetic tunnel junction(MTJ) element to be used as a memory element of a magnetic random accessmemory (MRAM).

FIGS. 1A to 1C are graphs showing comparison of the barrier oxidationtime dependence of the barrier resistance, barrier resistance change,and magnetoresistive ratio (MR ratio) of an MTJ element between an Rucap layer and a Ta cap layer. The Ru cap layer or Ta cap layer isprovided adjacent to one of the two ferromagnetic layers sandwiching thetunnel barrier layer of an MTJ element.

As is apparent from FIG. 5 (to be described later), even after annealingat 350° C., the Ru cap layer maintains magnetization of 90% or more thatbefore annealing. However, in the Ta cap layer, the magnetizationdecreases to 80% or less after annealing. When the Ru cap layer is used,element diffusion from the cap layer to the ferromagnetic layer is lowas compared to the Ta cap layer. That is, the Ru cap layer has a higherthermal stability than the Ta cap layer.

However, as shown in FIGS. 1A to 1C, when the Ru cap layer is used, thebarrier resistance of the MTJ element is high, and the barrierresistance change and the maximum value of the magnetoresistive ratio(MR ratio) are small as compared with the Ta cap layer.

Hence, a high thermal stability by low diffusion and a high MR ratio canhardly be implemented simultaneously by the Ru cap layer which placesemphasis on only preventing element diffusion or the Ta cap layer whichplaces emphasis on only preventing the decrease in MR ratio.

As a reason for the difference in MTJ characteristic between the Ru caplayer and the Ta cap layer, the difference in standard electrodepotential of the cap layer to an adjacent ferromagnetic layer will beexamined.

FIG. 2 shows the standard electrode potentials and reaction formulas ofvarious metal elements. FIGS. 3A and 3B are views showing models so asto explain the difference in barrier resistance and magnetoresistiveratio between cap layers. Referring to FIG. 2, in metal elements (e.g.,Ti, Fe, and the like) having two standard electrode potentials, thestandard electrode potential on the upper side is preferably used.

FIG. 2 indicates that as the standard electrode potential is close tothe (+) side, the Coulomb repulsion for metal ions to dissolve in anaqueous solution is necessary, and ionization hardly occurs.

Consider a case wherein the free layer adjacent to the Ru cap layer orTa cap layer is made of NiFe. In this case, the standard electrodepotentials of the materials of the cap layer and free layer are Ta (No.33), Fe (No. 34), Ni (No. 39), and Ru (No. 46) sequentially from the (−)side, as shown in FIG. 2. The ionization tendency increases in the orderof Ta, Ni, Fe, and Ru in accordance with the order of standard electrodepotentials. On the basis of the order of ionization tendencies, anexplanation can be done as shown in FIGS. 3A and 3B.

As shown in FIG. 3A, in the Ru cap layer, NiFe is ionized more readilythan Ru and charged to δ+. For this reason, in magnetic annealing aftertunneling magneto resistive (TMR) film formation, abnormal oxidation ofNiFe (the valence of the metal becomes positive) is promoted byrediffusion of the oxidation species from the tunnel barrier layer.Hence, the barrier resistance abnormally rises. The abnormal oxidationof NiFe also causes a decrease in steepness of the interface between theNiFe layer and the tunnel barrier layer. Hence, the barrier resistancechange and the maximum value of the MR ratio decrease.

On the other hand, as shown in FIG. 3B, in the Ta cap layer, Ta isionized more readily than NiFe, and NiFe is charged to δ−. For thisreason, abnormal oxidation of NiFe by rediffusion of the oxidationspecies from the tunnel barrier layer is suppressed. Hence, the increasein barrier resistance, the decrease in barrier resistance change, andthe decrease in maximum value of the MR ratio are prevented.

As described above, the low diffusion/high thermal stability and thehigh MR ratio cannot be implemented simultaneously by the Ru cap layerwhich places emphasis on preventing element diffusion from the cap layerto the ferromagnetic layer or the Ta cap layer which places emphasis onpreventing the decrease in MR ratio by preventing abnormal oxidation ofthe free layer.

Jpn. Pat. Appln. KOKAI Publication No. 2005-032780 and U.S. Pre-GrantPublication No. 2005/0008849 disclose a method of preventing oxidationof a free layer, in which a cap layer adjacent to the free layer of amagnetoresistive element is formed by using a material containing anelement having a high bond energy to O (oxygen). However, neither typesnor concentrations of elements to be combined with the element havingthe high bond energy to O are disclosed. When an element having a highbond energy and a high ionization tendency is used, a high MR ratio canbe implemented. However, low diffusion/high thermal stability cannot beexpected unless appropriate elements are combined at an appropriateconcentration ratio. Additionally, the bond energy to O is a physicalproperty value which reflects the state of a metal oxide as a productafter reaction and is therefore less appropriate as an index todetermine the reactivity to O than an ionization tendency (standardelectrode potential) which reflects the state of a metal element beforereaction.

Jpn. Pat. Appln. KOKAI Publication No. 2002-208119 discloses that thecap layer of a pinned layer of a giant magneto resistive (GMR) head isformed by using at least one of Ta, W, and Ti. However, neither typesnor concentrations of elements to be combined with these materials aredisclosed. A high MR ratio and low diffusion/high thermal stabilitycannot be implemented simultaneously unless appropriate elements arecombined at an appropriate concentration ratio.

Jpn. Pat. Appln. KOKAI Publication No. 2002-050011 discloses that theupper layer of the pinned layer of a magnetic recording medium is formedby using a mixture of at least two materials selected from the groupconsisting of Au, Ag, Cu, Mo, W, Y, Ti, Pt, Zr, Hf, V, Nb, Ta, and Ru.However, neither types nor concentrations of elements to be combined aredisclosed. A high MR ratio and low diffusion/high thermal stabilitycannot be implemented simultaneously unless appropriate elements arecombined at an appropriate concentration ratio.

Jpn. Pat. Appln. KOKAI Publication No. 2001-331908 discloses that in theslider cutting margin of a thin-film magnetic head, a local cell isformed by a magnetic metal film (NiFe) and a thin film (Cu, Al, Zn, orFe) having a high ionization tendency, thereby preventing magnetic metalcorrosion. In the thin film, the thermal stability is not taken intoconsideration. Hence, although a high MR ratio can be implemented, lowdiffusion/high thermal stability cannot be implemented simultaneouslyunless appropriate elements are combined at an appropriate concentrationratio.

Jpn. Pat. Appln. KOKAI Publication No. 2004-172599 discloses a filmstructure including a nonmagnetic conductor/anti-diffusionstructure/magnetic layer of a magnetoresistive element in which theanti-diffusion structure is made of one of AlO_(X), MgO_(X), SiO_(X),TiO_(X), CaO_(X), LiO_(X), HfO_(X), AlN, AlNO, SiN, SiNO, TiN, TiNO, BN,TaN, HfNO, and ZrN, and the nonmagnetic conductor contains at least oneelement selected from Al, Cu, Ta, Ru, Zr, Ti, Mo, and W. However, if thenonmagnetic conductor is readily diffused, diffusion to the magneticlayer progresses through the anti-diffusion structure. In addition, theionization tendency of the nonmagnetic conductor charges the magneticlayer through the thin anti-diffusion structure and influences thepresence/absence of abnormal oxidation of the magnetic layer. Hence, ahigh MR ratio and low diffusion/high thermal stability cannot beimplemented simultaneously unless appropriate elements are combined forthe nonmagnetic conductor at an appropriate concentration ratio.

On the basis of the above-described examinations, the embodiments of thepresent invention will be described below with reference to theaccompanying drawing.

[1] Magnetoresistive Element

In the first to third embodiments of the present invention, caseswherein an MTJ element is used as a magnetoresistive element will bedescribed.

[1-1] First Embodiment

FIG. 4 is a schematic view showing an MTJ element according to the firstembodiment of the present invention. The outline of the structure of theMTJ element according to the first embodiment will be described below.

As shown in FIG. 4, an MTJ element 100 according to the first embodimentincludes a magnetization pinned layer (magnetic layer) 111 whosemagnetization is fixed, a free layer (magnetic layer) 113 whosemagnetization reverses, a tunnel barrier layer (nonmagnetic layer) 112sandwiched between the magnetization pinned layer 111 and the free layer113, and an alloy cap layer 114 in contact with the free layer 113.

The alloy cap layer 114 is formed of a nonmagnetic layer. The alloy caplayer 114 is made of an alloy of a first metal material M1 and a secondmetal material M2. A standard electrode potential V1 of the first metalmaterial M1 is lower than a standard electrode potential V of the freelayer 113 adjacent to the alloy cap layer 114 (the ionization tendencyis high). A standard electrode potential V2 of the second metal materialM2 is higher than the standard electrode potential V (the ionizationtendency is low).

In the first embodiment, as the first metal material M1 of the alloy caplayer 114, a material having the standard electrode potential V1 lowerthan the standard electrode potential V of the free layer 113 adjacentto the alloy cap layer 114 (a material having a high ionizationtendency) is used, thereby preventing abnormal oxidation of the freelayer 113 and increasing the MR ratio. On the other hand, as the secondmetal material M2 of the alloy cap layer 114, a material having thestandard electrode potential V2 higher than the standard electrodepotential V of the free layer 113 adjacent to the alloy cap layer 114 (amaterial having a low ionization tendency) is used, thereby preventingelement diffusion from the alloy cap layer 114 to the free layer 113 andimproving the thermal stability. That is, in the first embodiment, botha high MR ratio and a high thermal stability are implemented by using analloy of the first metal material M1 which contributes to improvement ofthe MR ratio and the second metal material M2 which contributes toimprovement of the thermal stability.

The material and standard electrode potential of the alloy cap layer 114will be described below in detail.

(a) Material of Alloy Cap Layer

As described above, as the material of the alloy cap layer 114, an alloycontaining the first metal material M1 and the second metal material M2is used. The alloy materials and mixing ratio of the alloy cap layer 114containing the first and second metal materials M1 and M2 will bedescribed below in detail on the basis of the viewpoint of improvingboth the thermal stability and the MR ratio.

FIG. 5 is a graph showing the Ru—Ta or Ru—Cr composition dependence ofthe magnetization change in the free layer upon annealing when an Ru—Taor Ru—Cr alloy cap layer is used as the alloy cap layer according to thefirst embodiment of the present invention. FIG. 5 shows a resultobtained by using blanket wafer samples of an Ru—Ta or Ru—Cr alloy caplayer (thickness: 3 nm)/NiFe magnetic layer (thickness: 4nm)/AlO_(X)(thickness: 1 nm)/Ta (thickness: 3 nm)/substrate and checkingthe Ru—Ta or Ru—Cr composition dependence of the magnetization change inthe free layer (NiFe magnetic layer) upon annealing at 350° C. by avibrating sample magnetometer (VSM). The abscissa of FIG. 5 indicatesthat the mixing ratio of Ru increases leftward so that an Ru pure metalis obtained at the left end, and the mixing ratio of Ta or Cr increasesrightward so that a Ta or Cr pure metal is obtained at the right end.

As is apparent from FIG. 5, in the cap layer made of a Ta or Cr puremetal, diffusion of the cap layer material (Ta or Cr) to the free layerprogresses upon annealing, and the magnetization change in the freelayer decreases. However, when the mixing ratio of Ru with a highionization tendency to Ta or Cr with a low ionization tendency isincreased, diffusion of the cap layer material to the free layer issuppressed by the effect of Ru with low diffusion. Hence, thedegradation in magnetization caused by annealing is suppressed.

When the ratio of Ta in the Ru—Ta alloy cap layer is about 0.5 or less,or the ratio of Cr in the Ru—Cr alloy cap layer is about 0.3 or less,the degradation in magnetization caused by annealing is suppressed toalmost the same as in an Ru pure metal cap layer, and almost the samethermal stability as in the Ru pure metal cap layer is obtained.

When the ratio of Ta in the Ru—Ta alloy cap layer is 0.7, or the ratioof Cr in the Ru—Cr alloy cap layer is 0.5, the degradation inmagnetization progresses more than the case wherein the ratio of Ta orCr is lower. However, the magnetization change is smaller than in the Taor Cr pure metal cap layer. Hence, the degradation in magnetizationimproves as compared to the Ta or Cr pure metal cap layer.

Note that when the ratio of Cr in the Ru—Cr alloy cap layer is 0.7, themagnetization change is larger than in the Cr pure metal cap layer, andthe degradation in magnetization is promoted.

FIG. 6 is a graph showing the Ru—Ta composition dependence of thebarrier resistance and magnetoresistive ratio of an MTJ element when anRu—Ta alloy cap layer is used as the alloy cap layer according to thefirst embodiment of the present invention. The abscissa of FIG. 6indicates that the mixing ratio of Ru increases leftward so that an Rupure metal is obtained at the left end, and the mixing ratio of Taincreases rightward so that a Ta pure metal is obtained at the rightend.

As is apparent from FIG. 6, as the mixing ratio of Ta to Ru increases,the barrier resistance decreases, and the magnetoresistive ratioincreases. This is probably because the increase in barrier resistanceor the decrease in magnetoresistive ratio caused by abnormal oxidationof the free layer is suppressed by mixing Ta with a high ionizationtendency in Ru with a low ionization tendency (see the model shown inFIG. 3B).

As is apparent from FIGS. 5 and 6, in, e.g., the Ru—Ta alloy cap layer,it is especially desirable that both the high thermal stability and thehigh MR ratio are implemented when the ratio of Ta to Ru falls withinthe range of 0 to about 0.5. Hence, to implement the high thermalstability and the high MR ratio, the alloy cap layer 114 is necessary,in which a metal (Ru) which has a low ionization tendency, lowreactivity, and low diffusion and a metal (Ta) which has a highionization tendency and prevents abnormal oxidation of the free layerare present at an appropriate mixing ratio. That is, a metal with a highionization tendency supposedly has an effect of increasing the MR ratioby preventing abnormal oxidation of the magnetic layer. A metal with alow ionization tendency generally has a low reactivity and thereforesupposedly has an effect of preventing diffusion to the magnetic layer.

More specifically, the materials and mixing ratio of the alloy cap layer114 made of the first and second metal materials M1 and M2 to implementboth the high thermal stability and the high MR ratio are as follows.

The first metal material M1 of the alloy cap layer 114 preferablycontains a metal which has a high ionization tendency and preventsabnormal oxidation of the free layer 113, i.e., at least one elementselected from the group (first element group) consisting of, e.g., Ti,V, Cr, Mn, Zn, Zr, Nb, Hf, Ta, Fe, and Co.

The second metal material M2 of the alloy cap layer 114 preferablycontains a metal which has a low ionization tendency, low reactivity,and low diffusion, i.e., at least one element selected from the group(second element group) consisting of, e.g., Co, Ni, Cu, Mo, Ru, Rh, Pd,Ag, W, Re, Os, Ir, Pt, and Au.

To enable coexistence of the MR ratio increasing effect and the lowdiffusion/high thermal stability effect by the elements of the first andsecond element groups, the mixing ratio of the first metal material M1and second metal material M2 of the alloy cap layer 114 is preferablyset such that the abundance ratio of the element of the first elementgroup to the element of the second element group falls within the rangeof 1:99 to 99:1.

The ionization tendency of a magnetic element increases in the order ofFe, Co, and Ni. For this reason, Fe of the first element group isapplied when the major component of the magnetic layer is Co or Ni. Coof the first element group is applied when the major component of themagnetic layer is Ni. On the other hand, Co of the second element groupis applied when the major component of the magnetic layer is Fe. Ni ofthe second element group is applied when the major component of themagnetic layer is Fe or Co. The elements of the first and second elementgroups may appropriately be selected from the elements shown in FIG. 2.

As the first and second metal materials M1 and M2 of the alloy cap layer114, a combination of an element which has a relatively high meltingpoint and is hard to diffuse to the free layer 113 in the first elementgroup and a noble metal element which has an especially low reactivityin the second element group is used more preferably. More specifically,the alloy cap layer 114 preferably contains Ti—Cu, Ti—Ru, Ti—Rh, Ti—Pd,Ti—Ag, Ti—Ir, Ti—Pt, Ti—Au, Zr—Cu, Zr—Ru, Zr—Rh, Zr—Pd, Zr—Ag, Zr—Ir,Zr—Pt, Zr—Au, Nb—Cu, Nb—Ru, Nb—Rh, Nb—Pd, Nb—Ag, Nb—Ir, Nb—Pt, Nb—Au,Hf—Cu, Hf—Ru, Hf—Rh, Hf—Pd, Hf—Ag, Hf—Ir, Hf—Pt, or Hf—Au.

The main object of the alloy cap layer 114 is to protect magnetizationof the free layer 113. Hence, the alloy cap layer 114 is preferably madeof a nonmagnetic material so damage in the element formation processdoes not influence magnetization of the free layer 113.

(b) Standard Electrode Potential of Alloy Cap Layer

As described above, the standard electrode potentials V1 and V2 of themetal materials M1 and M2 contained in the alloy cap layer 114preferably satisfy relationships V1<V, and V2>V with respect to thestandard electrode potential V of the free layer 113 adjacent to thealloy cap layer 114. A standard electrode potential Va of the alloy ofthe alloy cap layer 114 will be described here.

The standard electrode potential Va of the alloy is calculated from theweighted average of standard electrode potentials corresponding to theconcentrations of the constituent materials on the basis of the standardelectrode potentials (FIG. 2) of the metals contained in the alloy. Thestandard electrode potential Va of an alloy containing, e.g., Ru₈Ta₂ canbe obtained by+0.455×8/(8+2)−0.6×2/(8+2)=+0.244(V)  (1)It is simple and preferable to estimate the standard electrode potentialVa of the alloy in this way.

FIG. 7 is a graph showing the metal composition dependence of thestandard electrode potential (ionization tendency) of an Ru—Ta or Ru—Cralloy cap layer when the Ru—Ta or Ru—Cr alloy cap layer is used as thealloy cap layer according to the first embodiment of the presentinvention. The abscissa of FIG. 7 indicates that the mixing ratio of Ruincreases leftward so that an Ru pure metal is obtained at the left end,and the mixing ratio of Ta or Cr increases rightward so that a Ta or Crpure metal is obtained at the right end.

The standard electrode potential Va of the alloy cap layer 114 toimprove the thermal stability is estimated on the basis ofcorrespondence of FIGS. 5 and 7. As shown in FIG. 7, in both the Ru—Taalloy cap layer and the Ru—Cr alloy cap layer, the degradation inmagnetization improves when the difference between the standardelectrode potential Va of the Ru—Ta or Ru—Cr alloy cap layer 114 and thestandard electrode potential V of the free layer 113 made of Ni₈Fe₂ isabout −0.2 V or more. When the ratio of Cr in the Ru—Cr alloy cap layer114 is 0.7, magnetization degrades more than that in a Cr pure metal caplayer. In this case, the difference between the standard electrodepotential Va of the alloy cap layer 114 and the standard electrodepotential V of the free layer 113 made of Ni₈Fe₂ is about −0.2 V. Hence,the difference between the standard electrode potential V of the freelayer 113 adjacent to the alloy cap layer 114 and the standard electrodepotential Va of the alloy of the alloy cap layer 114 is preferably −0.2V or more.

The standard electrode potential Va of the alloy cap layer 114 toincrease the MR ratio is estimated on the basis of correspondence ofFIGS. 6 and 7. As shown in FIG. 7, the difference between the standardelectrode potential of the Ru pure metal cap layer having a low MR ratioand the standard electrode potential V of the free layer 113 made ofNi₈Fe₂ is about +0.8 V. To increase the MR ratio, the standard electrodepotential Va of the alloy cap layer 114 is preferably lower (theionization tendency is preferably higher). Hence, the difference betweenthe standard electrode potential V of the free layer 113 adjacent to thealloy cap layer 114 and the standard electrode potential Va of the alloyof the alloy cap layer 114 is preferably +0.8 V or less.

To implement the high MR ratio and the high thermal stabilitysimultaneously, the difference between the standard electrode potentialV of the free layer 113 adjacent to the alloy cap layer 114 and thestandard electrode potential Va of the alloy of the alloy cap layer 114is preferably −0.2 V (inclusive) to +0.8 V (inclusive) in considerationof above-described two points.

FIGS. 8 to 15 are graphs showing the relationship between the metalcomposition ratio and the standard electrode potentials of detailedalloy cap layers according to the first embodiment of the presentinvention. As the free layer 113 adjacent to the alloy cap layer 114,three types of layers made of Ni₈Fe₂ (standard electrode potential:−0.295 V), Ni (standard electrode potential: −0.257 V), and Fe (standardelectrode potential: −0.447 V) are used.

FIG. 8 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Ru is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Ru—Ti, Ru—Hf, Ru—Zr, or Ru—Nb is used.

FIG. 9 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Cu is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Cu—Ti, Cu—Hf, Cu—Zr, or Cu—Nb is used.

FIG. 10 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Rh is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Rh—Ti, Rh—Hf, Rh—Zr, or Rh—Nb is used.

FIG. 11 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Ag is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Ag—Ti, Ag—Hf, Ag—Zr, or Ag—Nb is used.

FIG. 12 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Pd is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Pd—Ti, Pd—Hf, Pd—Zr, or Pd—Nb is used.

FIG. 13 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Ir is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Ir—Ti, Ir—Hf, Ir—Zr, or Ir—Nb is used.

FIG. 14 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Pt is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Pt—Ti, Pt—Hf, Pt—Zr, or Pt—Nb is used.

FIG. 15 shows a case wherein Ti, Hf, Zr, and Nb are selected from thefirst element group as the first metal material M1 of the alloy caplayer 114, and Au is selected from the second element group as thesecond metal material M2 of the alloy cap layer 114 so that the alloycap layer 114 made of Au—Ti, Au—Hf, Au—Zr, or Au—Nb is used.

FIGS. 8 to 15 show preferable ranges of the standard electrode potentialVa of the alloy of the alloy cap layer 114 to implement both the high MRratio and the high thermal stability. When the composition of the freelayer 113 changes, and the weighted average of the standard electrodepotential V of the free layer 113 changes, the preferable range of thestandard electrode potential Va of the alloy cap layer 114 also changes.However, independently of the material selected for the free layer 113adjacent to the alloy cap layer 114, both the high MR ratio and the highthermal stability can be implemented simultaneously when the differencebetween the standard electrode potential Va of the alloy of the alloycap layer 114 and the standard electrode potential V of the free layer113 is −0.2 V (inclusive) to +0.8 V (inclusive).

(c) Example

EXAMPLE 1

FIG. 16 is a sectional view of an MTJ element according to Example 1 ofthe present invention. The structure of the MTJ element according toExample 1 will be described below.

As shown in FIG. 16, an MTJ element 100 according to Example 1 includesa lower interconnection connecting layer 121 made of Ta (thickness: 5nm), a buffer layer 122 made of Ru (thickness: 1 nm), anantiferromagnetic layer 123 made of Pt—Mn (thickness: 15 nm), amagnetization pinned layer 111 made of Co—Fe (thickness: 5 nm), a tunnelbarrier layer 112 made of aluminum oxide (AlO_(X)) (thickness: 1 nm), afree layer 113 made of Ni—Fe (thickness: 4 nm), an alloy cap layer 114made of an Ru—Ta alloy or Ru—Cr alloy (thickness: 3 nm), and a masklayer 124 made of Ta (thickness: 50 nm). The mask layer 124 functions asan etching mask, surface protection layer, and upper interconnectionconnecting layer.

EXAMPLE 2

FIG. 17 is a sectional view of an MTJ element according to Example 2 ofthe present invention. The structure of the MTJ element according toExample 2 will be described below.

As shown in FIG. 17, an MTJ element 100 according to Example 2 has aso-called synthetic ferrimagnetic pinned layer structure in which amagnetization pinned layer 111 includes a ferromagnetic layer 111a/nonmagnetic layer 111 b/ferromagnetic layer 111 c. The ferromagneticlayers 111 a and 111 c are antiferromagnetically coupled.

More specifically, the MTJ element 100 includes a lower interconnectionconnecting layer 121 made of Ta (thickness: 5 nm), an antiferromagneticlayer 123 made of Pt—Mn (thickness: 15 nm), the magnetic layer 111 amade of Co—Fe (thickness: 2 nm), the nonmagnetic layer 111 b made of anRu—Ta alloy, the magnetic layer 111 c made of Co—Fe (thickness: 2 nm), atunnel barrier layer 112 made of aluminum oxide (AlO_(X)) (thickness: 1nm), a free layer 113 made of Ni—Fe (thickness: 4 nm), an alloy caplayer 114 made of an Ru—Ta alloy or Ru—Cr alloy (thickness: 3 nm), and amask layer 124 made of Ta (thickness: 50 nm).

Normally, a metal such as Ru having a low ionization tendency is used asthe nonmagnetic layer 111 b in the magnetization pinned layer 111 withthe synthetic ferrimagnetic pinned layer structure. In Example 2, analloy is used, like the alloy cap layer 114. When a metal having a highionization tendency is added to even the nonmagnetic layer 111 b at aconcentration ratio not to cause any degradation in thermal stability ormagnetic characteristic of the synthetic ferrimagnetic pinned layer,abnormal oxidation of the interface between the magnetic layer 111 c andthe tunnel barrier layer 112 is prevented, and the MR ratio increases.

Example 3

FIG. 18 is a sectional view of an MTJ element according to Example 3 ofthe present invention. The structure of the MTJ element according toExample 3 will be described below.

As shown in FIG. 18, an MTJ element 100 according to Example 3 has atop-pin structure in which a magnetization pinned layer 111 is arrangedon a tunnel barrier layer 112, not a bottom-pin structure in which themagnetization pinned layer 111 is arranged under the tunnel barrierlayer 112 as in Examples 1 and 2.

More specifically, the MTJ element 100 includes a lower interconnectionconnecting layer 121 made of Ta (thickness: 5 nm), a lower layer 125 ofa free layer, which is made of an Ru—Ta alloy or Ru—Cr alloy (thickness:3 nm), a free layer 113 made of Ni—Fe (thickness: 4 nm), a tunnelbarrier layer 112 made of aluminum oxide (AlO_(X)) (thickness: 1 nm), amagnetic layer 111 c made of Co—Fe (thickness: 2 nm), a nonmagneticlayer 111 b made of an Ru—Ta alloy, a magnetic layer 111 a made of Co—Fe(thickness: 2 nm), an antiferromagnetic layer 123 made of Pt—Mn(thickness: 15 nm), and a mask layer 124 made of Ta (thickness: 50 nm).

The same alloy layer as the above-described alloy cap layer 114 isapplied to the lower layer 125 of the free layer and the nonmagneticlayer 111 b. With this structure, abnormal oxidation of the interfacebetween the tunnel barrier layer 112 and the free layer 113 and theinterface between the tunnel barrier layer 112 and the magnetic layer111 c is prevented. In addition, element diffusion from the lower layer125 of the free layer to the free layer 113 and element diffusion fromthe nonmagnetic layer 111 b to the magnetic layer 111 c are suppressed.Hence, both the MR ratio and the thermal stability of the MTJ element100 can be improved.

Example 4

FIGS. 19A to 19F are sectional views showing steps in manufacturing aportion near a memory cell of an MTJ element according to Example 4 ofthe present invention. The method of manufacturing the portion near thememory cell of the MTJ element will be described below. Layers under thealloy cap layer 114 in FIG. 16 or 17 are simply shown as a lower layer202 of the cap layer.

As shown in FIG. 19A, a lower interconnection layer 201 is formed on asubstrate (not shown). Examples of the material of the lowerinterconnection layer 201 are Al, Al—Cu, Cu, Ta, W, and Ag. The lowerlayer 202 of the cap layer, an alloy cap layer 114 made of Ru—Ta, Ru—Cr,or Ni—Fe—Zr, and a mask layer 124 are sequentially formed on the lowerinterconnection layer 201 by, e.g., high-vacuum sputtering. Instead ofhigh-vacuum sputtering, deposition, chemical vapor deposition (CVD), oratomic layer deposition (ALD) may be used. In forming the barrier layerin the lower layer 202 of the cap layer, a metal (e.g., Al) to beoxidized/nitrided may be deposited and then oxidized by using oxygenplasma, oxygen radical, ozone, or oxygen gas atmosphere or nitrided byusing nitrogen plasma or nitrogen radical.

As shown in FIG. 19B, the lower layer 202 of the cap layer, the alloycap layer 114, and the mask layer 124 are selectively etched by, e.g.,ion milling.

As shown in FIG. 19C, an insulating film 203 to protect an MTJ element100 in the next step is formed by, e.g., sputtering or chemical vapordeposition (CVD). As the material of the insulating film 203, forexample, a silicon oxide film (SiO_(X)) or silicon nitride film(SiN_(X)) is used.

Next, the lower interconnection layer 201 is selectively etched by,e.g., reactive ion etching (RIE). Although not illustrated, processedportions of the lower interconnection layer 201 are present on, e.g.,the near and far sides of the drawing surface of FIG. 19C. At this time,the MTJ element 100 is protected by the insulating film 203 shown inFIG. 19C.

As shown in FIG. 19D, an insulating film 204 is formed on the insulatingfilm 203 by, e.g., sputtering or chemical vapor deposition. As thematerial of the insulating film 204, for example, a silicon oxide film(SiO_(X)) or silicon nitride film (SiN_(X)) is used. The insulating film204 is selectively etched to form a contact hole 205 to electricallyconnect the mask layer 124 and lower interconnection layer 201. Althoughnot illustrated, contact holes to the lower interconnection layer 201are formed on, e.g., the left and right outer sides of FIG. 19D.

As shown in FIG. 19E, an upper interconnection layer 206 is formed onthe insulating film 204 and in the contact hole 205. Examples of thematerial of the upper interconnection layer 206 are Al, Al—Cu, Cu, Ta,W, and Ag.

Finally, as shown in FIG. 19F, the upper interconnection layer 206 isselectively etched by, e.g., reactive ion etching. Formation of theportion near the memory cell is thus ended.

As described above, according to the first embodiment, the alloy caplayer 114 is provided adjacent to the free layer 113 of the MTJ element100. The alloy cap layer 114 is made of the first metal material M1having the standard electrode potential V1 lower than the standardelectrode potential V of the free layer 113 (the ionization tendency ishigh) and the second metal material M2 having the standard electrodepotential V2 higher than the standard electrode potential V of the freelayer 113 (the ionization tendency is low). When the alloy cap layer 114is used, the free layer 113 is charged to δ− by the function of thefirst metal material M1 having the high ionization tendency. Hence,abnormal oxidation of the free layer 113, which is caused by rediffusionof the oxidation species in magnetic annealing after TMR film formation,is suppressed, and the magnetoresistive ratio increases. On the otherhand, element diffusion from the alloy cap layer 114 to the free layer113 is suppressed by the effect of the second metal material M2 having alow reactivity and low ionization tendency, and a high thermal stabilityis obtained as an MTJ characteristic. When the above-described twoeffects are simultaneously obtained by the alloy cap layer 114, both thehigh thermal stability and the high MR ratio can be implemented.

In the first embodiment, the thickness of each layer of the MTJ element100 may appropriately be adjusted within the range of several Å toseveral tens of nm. As the material of each layer of the MTJ element100, a material different from those described above may be used. Thestructure may also be reversed. As the tunnel barrier layer 112, MgO,AlN, AlON, AlHfO_(X), AlZrO_(X), or AlFO_(X) may be used. Aferromagnetic double tunnel junction structure including a plurality oftunnel barrier layers 112 may be employed. The free layer 113 need notalways have a single-layer structure and may have a multilayer structureincluding a ferromagnetic layer/nonmagnetic layer/ferromagnetic layer.

[1-2] Second Embodiment

The second embodiment is different from the first embodiment in thematerial of the alloy cap layer. A magnetic material is used for thealloy cap layer.

FIG. 20 is a schematic view showing an MTJ element according to thesecond embodiment of the present invention. The outline of the structureof the MTJ element according to the second embodiment will be describedbelow.

As shown in FIG. 20, an MTJ element 100 according to the secondembodiment includes a magnetization pinned layer (magnetic layer) 111whose magnetization is fixed, a free layer (magnetic layer) 113 whosemagnetization reverses, a tunnel barrier layer (nonmagnetic layer) 112sandwiched between the magnetization pinned layer 111 and the free layer113, and an alloy cap layer 114 in contact with the free layer 113, asin the first embodiment.

The alloy cap layer 114 is made of an alloy of a metal material M3 and amagnetic material M4. A standard electrode potential V3 of the metalmaterial M3 is lower than a standard electrode potential V of the freelayer 113 adjacent to the alloy cap layer 114 (the ionization tendencyis high).

FIG. 21 is a graph showing the NiFe—Zr composition dependence of thebarrier resistance and magnetoresistive ratio of the MTJ element when anNiFe—Zr alloy cap layer is used as the alloy cap layer according to thesecond embodiment of the present invention. The abscissa of FIG. 21indicates that the mixing ratio of NiFe increases leftward so that analloy containing only NiFe is obtained at the left end, and the mixingratio of Zr increases rightward so that a Zr pure metal is obtained atthe right end.

The alloy cap layer 114 made of Ni—Fe—Zr uses Zr as the metal materialM3 and Ni—Fe as the magnetic material M4. The standard electrodepotential V3 of Zr is lower than that of Ni or Fe contained in the freelayer 113 so that the ionization tendency is high. When the Zrconcentration in the alloy cap layer 114 is high, abnormal oxidation ofthe free layer 113 is prevented (see the model shown in FIG. 3B), thebarrier resistance decreases, and the magnetoresistive ratio increases.In this case, no metal having a lower ionization tendency than Ni or Feof the free layer 113 is present in the alloy cap layer 114. Hence, themetal material M3 having a high ionization tendency may be diffused fromthe alloy cap layer 114 to the free layer 113 upon annealing. However,since the magnetic material M4 is also diffused from the alloy cap layer114 simultaneously, the magnetization change in the free layer 113 isminimized. Hence, the MTJ element 100 having a high thermal stabilityand a high MR ratio is implemented.

The materials of the alloy cap layer 114 made of the magnetic materialM4 and the metal material M3 having an ionization tendency higher thanthe free layer 113 are as follows.

The magnetic material M4 of the alloy cap layer 114 preferably containsat least one element selected from the group (third element group)consisting of Co, Fe, and Ni.

The metal material M3 of the alloy cap layer 114, which has a highionization tendency (negative standard electrode potential), preferablycontains at least one element selected from the group (fourth elementgroup) consisting of Ti, V, Cr, Mn, Zn, Zr, Nb, Hf, and Ta.

To enable coexistence of the effects of elements of both groups, theabundance ratio of the element of the fourth element group to theelement of the third element group preferably falls within the range of1% to 99%.

As the magnetic material M4 and metal material M3 of the alloy cap layer114, an element which has a relatively high melting point and is hard todiffuse to the free layer 113 in the fourth element group is usedpreferably. More specifically, the alloy cap layer 114 preferablycontains Co—Ti, Co—Zr, Co—Nb, Co—Hf, Fe—Ti, Fe—Zr, Fe—Nb, Fe—Hf, Ni—Ti,Ni—Zr, Ni—Nb, or Ni—Hf. The elements of the fourth element group mayappropriately be selected from the elements shown in FIG. 2.

More preferably, the alloy cap layer 114 uses an alloy of the magneticmetal M4 which is the same as the major component of the free layer 113and the metal material M3 whose standard electrode potential is locatedon the negative side of that of the magnetic metal M4 and whosesolubility limit to the magnetic metal M4 is 5% or less. When theconcentration of the metal of the metal material M3 is higher than 5%,the metal elutes to the free layer 113 in the hot process. However,since the magnetic metal M4 which is the same as the major component ofthe free layer 113 also elutes simultaneously, the magnetization changein the free layer 113 is further suppressed.

More specifically, the alloy cap layer 114 preferably contains two ofthe magnetic elements Co, Fe, and Ni, which have the first and secondlargest contents in the free layer 113, and at least one element of Zr,Nb, Hf, and Ti.

For example, when the elements having the first and second largestcontents in the free layer 113 are Co and Fe, the alloy cap layer 114preferably contains Co—Fe—Zr, Co—Fe—Nb, Co—Fe—Hf, or Co—Fe—Ti. When theelements having the first and second largest contents in the free layer113 are Co and Ni, the alloy cap layer 114 preferably contains Co—Ni—Zr,Co—Ni—Nb, Co—Ni—Hf, or Co—Ni—Ti. When the elements having the first andsecond largest contents in the free layer 113 are Fe and Ni, the alloycap layer 114 preferably contains Ni—Fe—Zr, Ni—Fe—Nb, Ni—Fe—Hf, orNi—Fe—Ti.

As described above, according to the second embodiment, the alloy caplayer 114 is provided adjacent to the free layer 113 of the MTJ element100. The alloy cap layer 114 is made of the magnetic material M4 and themetal material M3 having the standard electrode potential V3 lower thanthe standard electrode potential V of the free layer 113 (the ionizationtendency is high). When the alloy cap layer 114 is used, the free layer113 is charged to δ− by the function of the metal M3 having the highionization tendency. Hence, abnormal oxidation of the free layer 113,which is caused by rediffusion of the oxidation species in magneticannealing after TMR film formation, is suppressed, and themagnetoresistive ratio increases. On the other hand, since the magneticmaterial M4 is diffused to the free layer 113 together with the metalmaterial M3 having the high ionization tendency, the decrease inmagnetization of the free layer 113 is suppressed, and a high thermalstability is obtained as an MTJ characteristic. When the above-describedtwo effects are simultaneously obtained by the alloy cap layer 114, boththe high thermal stability and the high MR ratio can be implemented.

In the second embodiment, to implement the high thermal stability andthe high MR ratio simultaneously, the difference between a standardelectrode potential Vb of the alloy of the alloy cap layer 114 made ofthe magnetic material M4 and metal material M3 and the standardelectrode potential V of the adjacent free layer 113 is also preferably−0.2 V (inclusive) to +0.8 V (inclusive), as in the first embodiment.

[1-3] Third Embodiment

In the third embodiment, a diffusion suppressing layer is providedbetween an alloy cap layer and a magnetic layer adjacent to the alloycap layer in the first and second embodiments.

FIG. 22 is a schematic view showing an MTJ element according to thethird embodiment of the present invention. The outline of the structureof the MTJ element according to the third embodiment will be describedbelow.

As shown in FIG. 22, in an MTJ element 100 according to the thirdembodiment, a diffusion suppressing layer 115 is provided between analloy cap layer 114 and a free layer 113 to enhance the anti-diffusionfunction from the alloy cap layer 114 to the free layer 113.

The diffusion suppressing layer 115 is made of a metal oxide, metalnitride, or metal oxynitride. More specifically, the diffusionsuppressing layer 115 is made of an oxide, nitride, or oxynitride of anelement containing at least one element selected from the group (fifthelement group) consisting of, e.g., Al, Mg, Cr, V, B, W, Ti, Zr, Hf, andTa. Preferably, a relatively stable oxide or nitride is used. The effectis especially large in, e.g., AlO_(X), MgO, CrO_(X), VO_(X), BN, WN,TiN, ZrO_(X), ZrN, HfO_(X), HfN, TaO_(X), or TaN.

The diffusion suppressing layer 115 can be either an insulating orconductive layer. If the diffusion suppressing layer 115 has insulatingproperties, its thickness is preferably as small as about 2 nm or lessto maintain the effect of preventing abnormal oxidation by exchangingcharges between the alloy cap layer 114 and the free layer 113 andcharging the free layer 113 to δ−. In addition, the diffusionsuppressing layer 115 is preferably nonmagnetic.

As described above, according to the third embodiment, the same effectas in the first and second embodiments can be obtained. In addition,when the diffusion suppressing layer 115 is provided between the alloycap layer 114 and the free layer 113, the function of preventingdiffusion from the alloy cap layer 114 to the free layer 113 can furtherbe increased.

[2] Magnetic Random Access Memory

A magnetic random access memory according to an embodiment of thepresent invention will be described next. In this magnetic random accessmemory, the MTJ element 100 having the above-described alloy cap layer114 is used as the memory element of a memory cell. As examples of thememory cell structure of the magnetic random access memory, [2-1] selecttransistor cell, [2-2] select diode cell, [2-3] cross-point cell, and[2-4] toggle cell will be described here. [2-1] Select Transistor Cell

FIGS. 23A and 23B show select transistor memory cells of the magneticrandom access memory according to the embodiment of the presentinvention. The transistor cell structure will be described below.

As shown in FIGS. 23A and 23B, one cell MC having a select transistorstructure includes one MTJ element 100, a transistor (e.g., a MOStransistor) Tr connected to the MTJ element 100, a bit line (BL) 28, anda write word line (WWL) 26. A memory cell array MCA is formed by layingout a plurality of memory cells MC in an array.

More specifically, one terminal of the MTJ element 100 is connected toone end (drain diffusion layer) 23 a of the current path of thetransistor Tr through a base metal layer 27, contacts 24 a, 24 b, and 24c, and interconnections 25 a and 25 b. The other terminal of the MTJelement 100 is connected to the bit line 28. The write word line 26electrically disconnected from the MTJ element 100 is provided under theMTJ element 100. The other end (source diffusion layer) 23 b of thecurrent path of the transistor Tr is connected to, e.g., ground througha contact 24 d and interconnection 25 c. A gate electrode 22 of thetransistor Tr functions as a read word line (RWL).

One terminal of the MTJ element 100 on the side of the base metal layer27 is, e.g., a magnetization pinned layer 111. The other terminal of theMTJ element 100 on the side of the bit line 28 is, e.g., an alloy caplayer 114. The arrangement may be reversed, as a matter of course. Theaxis of easy magnetization of the MTJ element 100 can be arranged invarious directions with respect to the running direction of the writeinterconnection. For example, the axis of easy magnetization can bearranged either in the running direction of the bit line 28 or in therunning direction of the write word line 26.

In the above-described select transistor memory cell, the data write andread are executed in the following way.

The write operation is executed in the following way. The bit line 28and write word line 26 corresponding to a selected one of the pluralityof MTJ elements 100 are selected. Write currents Iw1 and Iw2 aresupplied to the selected bit line 28 and write word line 26,respectively. A synthetic field H by the write currents Iw1 and Iw2 isapplied to the MTJ element 100. The magnetization of a free layer 113 ofthe MTJ element 100 is reversed so that a state wherein themagnetization directions of the magnetization pinned layer 111 and freelayer 113 are parallel or a state wherein the magnetization directionsare anti-parallel is set. When the parallel state is defined as, e.g., a“1” state, and the anti-parallel state is defined as a “0” state, abinary data write is implemented.

The read operation is executed in the following way by using thetransistor Tr which functions as a read switching element. The bit line28 and read word line (RWL) corresponding to the selected MTJ element100 are selected. A read current Ir which tunnels through a tunnelbarrier layer 112 of the MTJ element 100 is supplied. The junctionresistance value changes in proportion to the cosine of the relativeangle between the magnetization of the magnetization pinned layer 111and that of the free layer 113. When the magnetization of the MTJelement 100 is in the parallel state (e.g., “1” state), the resistanceis low. When the magnetization is in the anti-parallel state (e.g., “0”state), the resistance is high. That is, the tunnel magneto-resistance(TMR) effect is obtained. The “1” or “0” state of the MTJ element 100 isdiscriminated by reading the difference in resistance value.

[2-2] Select Diode Cell

FIGS. 24A and 24B show select diode memory cells of the magnetic randomaccess memory according to the embodiment of the present invention. Theselect diode cell structure will be described below.

As shown in FIGS. 24A and 24B, one cell MC having a select diodestructure includes one MTJ element 100, a diode D connected to the MTJelement 100, the bit line 28, and the word line (WL) 26. The memory cellarray MCA is formed by laying out a plurality of memory cells MC in anarray.

The diode D is, e.g., a p-n junction diode including a p-typesemiconductor layer and an n-type semiconductor layer. One terminal(e.g., p-type semiconductor layer) of the diode D is connected to theMTJ element 100. The other terminal (e.g., n-type semiconductor layer)of the diode D is connected to the word line 26. In the structure shownin FIGS. 24A and 24B, a current flows from the bit line 28 to the wordline 26.

The location or direction of the diode D can be changed variously. Forexample, the diode D may be arranged in a direction to supply a currentfrom the word line 26 to the bit line 28. The diode D may be formed in asemiconductor substrate 21. The diode D may have the same shape (e.g., aso-called cross shape) as the MTJ element 100.

The data write operation of the select diode memory cell is the same asthat of the above-described select transistor cell. The write currentsIw1 and Iw2 are supplied to the bit line 28 and word line 26 to set themagnetization of the MTJ element 100 in the parallel or anti-parallelstate.

The data read operation is also almost the same as that of the selecttransistor cell. In the select diode cell, the diode D is used as a readswitching element. More specifically, the biases of the bit line 28 andword line 26 are controlled by using the rectifying effect of the diodeD such that an unselected MTJ element 100 is reverse-biased.Accordingly, the read current Ir is supplied to only the selected MTJelement 100.

[2-3] Cross-Point Cell

FIGS. 25A and 25B show cross-point memory cells of the magnetic randomaccess memory according to the embodiment of the present invention. Thecross-point cell structure will be described below.

As shown in FIGS. 25A and 25B, one cell MC having a cross-pointstructure includes one MTJ element 100, the bit line 28, and the wordline 26. The memory cell array MCA is formed by laying out a pluralityof memory cells MC in an array.

More specifically, the MTJ element 100 is arranged near the intersectionbetween the bit line 28 and the word line 26. One terminal of the MTJelement 100 is connected to the word line 26. The other terminal of theMTJ element 100 is connected to the bit line 28.

The data write operation of the cross-point memory cell is the same asthat of the above-described select transistor cell. The write currentsIw1 and Iw2 are supplied to the bit line 28 and word line 26 to set themagnetization of the MTJ element 100 in the parallel or anti-parallelstate. In the data read operation, the read current Ir is supplied tothe bit line 28 and word line 26 connected to the selected MTJ element100, thereby reading out the data of the MTJ element 100.

[2-4] Toggle Cell

FIG. 26 is a plan view showing a toggle memory cell of the magneticrandom access memory according to the embodiment of the presentinvention. The toggle cell structure will be described below.

As shown in FIG. 26, in the toggle cell, the MTJ element 100 is arrangedsuch that the axis of easy magnetization of the MTJ element 100 istilted with respect to the running direction (X direction) of the bitline 28 or the running direction (Y direction) of the word line 26,i.e., tilted with respect to the direction of the write current Iw1 tobe supplied to the bit line 28 or the direction of the write current Iw2supplied to the word line 26. The tilt of the MTJ element 100 is, e.g.,about 30°0 to 60°, and preferably, about 45°. In the toggle cell, thefree layer 113 of the MTJ element 100 preferably has anantiferromagnetic coupling structure containing a ferromagneticlayer/nonmagnetic layer/ferromagnetic layer.

In the above-described toggle memory cell, the data write and read areexecuted in the following way.

The write operation is executed in the following way. In the togglewrite, before arbitrary data is written in the selected cell, the dataof the selected cell is read out. If it is determined by reading out thedata of the selected cell that the arbitrary data has already beenwritten, no write is executed. If data different from the arbitrary datais written, the write is executed to rewrite the data.

After the above-described check cycle, if data must be written in theselected cell, the two write interconnections (bit line 28 and word line26) are sequentially turned on. The write interconnection turned onfirst is turned off. Then, the write interconnection turned on later isturned off. For example, the procedures comprise four cycles: the wordline 26 is turned on to supply the write current Iw2→ the bit line 28 isturned on to supply the write current Iw1 → the word line 26 is turnedoff to stop supplying the write current Iw2→ the bit line 28 is turnedoff to stop supplying the write current Iw1.

In the data read operation, the read current Ir is supplied to the bitline 28 and word line 26 connected is to the selected MTJ element 100,thereby reading out the data of the MTJ element 100.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A magnetoresistive element comprising: a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential; a second magnetic layer; a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer; and a nonmagnetic cap layer which contacts thesecond surface of the first magnetic layer and is formed from an alloyof a first metal material and a second metal material, the first metalmaterial having a second standard electrode potential lower than thefirst standard electrode potential, the second metal material having athird standard electrode potential higher than the first standardelectrode potential.
 2. The element according to claim 1, wherein afourth standard electrode potential of the nonmagnetic cap layer is notless than −0.2 V with respect to the first standard electrode potentialand is not more than +0.8 V with respect to the first standard electrodepotential.
 3. The element according to claim 2, wherein the fourthstandard electrode potential is calculated from a weighted average ofthe second standard electrode potential and the third standard electrodepotential.
 4. The element according to claim 1, wherein the first metalmaterial contains an element selected from the group consisting of Ti,V, Cr, Mn, Zn, Zr, Nb, Hf, Ta, Fe, and Co, and the second metal materialcontains an element selected from the group consisting of Co, Ni, Cu,Mo, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt, and Au.
 5. A magnetoresistiveelement comprising: a first magnetic layer which includes a firstsurface and a second surface and has a first standard electrodepotential; a second magnetic layer; a barrier layer which is providedbetween the second magnetic layer and the first surface of the firstmagnetic layer; a nonmagnetic cap layer which is arranged on a side ofthe second surface of the first magnetic layer and is formed from analloy of a first metal material and a second metal material, the firstmetal material having a second standard electrode potential lower thanthe first standard electrode potential, the second metal material havinga third standard electrode potential higher than the first standardelectrode potential; and a diffusion suppressing layer which suppressesdiffusion from the nonmagnetic cap layer to the first magnetic layer andis provided between the nonmagnetic cap layer and the first magneticlayer, the diffusion suppressing layer containing one of a metal oxide,a metal nitride, and a metal oxynitride.
 6. The element according toclaim 5, wherein the diffusion suppressing layer contains one of anoxide, a nitride, and an oxynitride of an element containing an elementselected from the group consisting of Al, Mg, B, Ti, Zr, Hf, and Ta. 7.The element according to claim 5, wherein a fourth standard electrodepotential of the nonmagnetic cap layer is not less than −0.2 V withrespect to the first standard electrode potential and is not more than+0.8 V with respect to the first standard electrode potential.
 8. Theelement according to claim 7, wherein the fourth standard electrodepotential is calculated from a weighted average of the second standardelectrode potential and the third standard electrode potential.
 9. Theelement according to claim 5, wherein the first metal material containsan element selected from the group consisting of Ti, V, Cr, Mn, Zn, Zr,Nb, Hf, Ta, Fe, and Co, and the second metal material contains anelement selected from the group consisting of Co, Ni, Cu, Mo, Ru, Rh,Pd, Ag, W, Re, Os, Ir, Pt, and Au.
 10. A magnetoresistive elementcomprising: a first magnetic layer which includes a first surface and asecond surface and has a first standard electrode potential; a secondmagnetic layer; a barrier layer which is provided between the secondmagnetic layer and the first surface of the first magnetic layer; and anonmagnetic cap layer which contacts the second surface of the firstmagnetic layer and is formed from an alloy of a magnetic material and ametal material, the metal material having a second standard electrodepotential lower than the first standard electrode potential.
 11. Theelement according to claim 10, wherein the magnetic material contains anelement selected from the group consisting of Co, Fe, and Ni, and themetal material contains an element selected from the group consisting ofTi, V, Cr, Mn, Zn, Zr, Nb, Hf, and Ta.
 12. A magnetoresistive elementcomprising: a first magnetic layer which includes a first surface and asecond surface and has a first standard electrode potential; a secondmagnetic layer; a barrier layer which is provided between the secondmagnetic layer and the first surface of the first magnetic layer; anonmagnetic cap layer which is arranged on a side of the second surfaceof the first magnetic layer and is formed from an alloy of a magneticmaterial and a metal material, the metal material having a secondstandard electrode potential lower than the first standard electrodepotential; and a diffusion suppressing layer which suppresses diffusionfrom the nonmagnetic cap layer to the first magnetic layer and isprovided between the nonmagnetic cap layer and the first magnetic layer,the diffusion suppressing layer containing one of a metal oxide, a metalnitride, and a metal oxynitride.
 13. The element according to claim 12,wherein the diffusion suppressing layer contains one of an oxide, anitride, and an oxynitride of an element containing an element selectedfrom the group consisting of Al, Mg, B, Ti, Zr, Hf, and Ta.
 14. Theelement according to claim 12, wherein the magnetic material contains anelement selected from the group consisting of Co, Fe, and Ni, and themetal material contains an element selected from the group consisting ofTi, V, Cr, Mn, Zn, Zr, Nb, Hf, and Ta.
 15. A magnetic random accessmemory comprising: a magnetoresistive element as a memory element, themagnetoresistive element comprising: a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential; a second magnetic layer; a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer; and a nonmagnetic cap layer which contacts thesecond surface of the first magnetic layer and is formed from an alloyof a first metal material and a second metal material, the first metalmaterial having a second standard electrode potential lower than thefirst standard electrode potential, the second metal material having athird standard electrode potential higher than the first standardelectrode potential.
 16. A magnetic random access memory comprising: amagnetoresistive element as a memory element, the magnetoresistiveelement comprising: a first magnetic layer which includes a firstsurface and a second surface and has a first standard electrodepotential; a second magnetic layer; a barrier layer which is providedbetween the second magnetic layer and the first surface of the firstmagnetic layer; a nonmagnetic cap layer which is arranged on a side ofthe second surface of the first magnetic layer and is formed from analloy of a first metal material and a second metal material, the firstmetal material having a second standard electrode potential lower thanthe first standard electrode potential, the second metal material havinga third standard electrode potential higher than the first standardelectrode potential; and a diffusion suppressing layer which suppressesdiffusion from the nonmagnetic cap layer to the first magnetic layer andis provided between the nonmagnetic cap layer and the first magneticlayer, the diffusion suppressing layer containing one of a metal oxide,a metal nitride, and a metal oxynitride.
 17. A magnetic random accessmemory comprising: a magnetoresistive element as a memory element, themagnetoresistive element comprising: a first magnetic layer whichincludes a first surface and a second surface and has a first standardelectrode potential; a second magnetic layer; a barrier layer which isprovided between the second magnetic layer and the first surface of thefirst magnetic layer; and a nonmagnetic cap layer which contacts thesecond surface of the first magnetic layer and is formed from an alloyof a magnetic material and a metal material, the metal material having asecond standard electrode potential lower than the first standardelectrode potential.
 18. A magnetic random access memory comprising: amagnetoresistive element as a memory element, the magnetoresistiveelement comprising: a first magnetic layer which includes a firstsurface and a second surface and has a first standard electrodepotential; a second magnetic layer; a barrier layer which is providedbetween the second magnetic layer and the first surface of the firstmagnetic layer; a nonmagnetic cap layer which is arranged on a side ofthe second surface of the first magnetic layer and is formed from analloy of a magnetic material and a metal material, the metal materialhaving a second standard electrode potential lower than the firststandard electrode potential; and a diffusion suppressing layer whichsuppresses diffusion from the nonmagnetic cap layer to the firstmagnetic layer and is provided between the nonmagnetic cap layer and thefirst magnetic layer, the diffusion suppressing layer containing one ofa metal oxide, a metal nitride, and a metal oxynitride.